Positron emission computed tomography (PET) is a nuclear medicine functional imaging technique that is used to observe metabolic activities in a subject. A PET system has multiple components including a gantry assembly, a detector assembly, a cooling assembly, etc. Proper installation, maintenance, and protection of the multiple components may affect various aspects of performance of a PET system including, e.g., the imaging accuracy, service life, work efficiency, etc. It is desirable to improve the design of multiple components to facilitate the installation, maintenance, and protection of the PET system. Besides, a subject needs to be exposed to radiation in PET-CT imaging. Thus, it is desirable to improve the design of the PET-CT imaging system to reduce the radiation dose.